1. Field of the Invention
The present invention relates to a fuel comprising an emulsion between water and a liquid hydrocarbon. More particularly, the present invention relates to a fuel comprising an emulsion between water and a liquid hydrocarbon, this emulsion being stabilized by adding an emulsifier.
2. Description of the Related Art
It is known that the combustion of liquid hydrocarbons, for example for feeding internal combustion engines or for producing heat, leads to the formation of numerous pollutants, in particular soot, particulates, carbon monoxide (CO), nitrogen oxides (NOx), sulphur oxides (SOx), and noncombusted hydrocarbons, which contribute significantly towards atmospheric pollution.
It is also known that the addition of controlled amounts of water to fuel can significantly reduce the production of pollutants. It is believed that this effect is the result of various phenomena arising from the presence of water in the, combustion zone. For example, the lowering of the peak combustion temperature by water reduces the emission of nitrogen oxides (NOx), the formation of which is promoted by high temperatures. In addition, the instantaneous vaporization of the water promotes better dispersion of the fuel in the combustion chamber, thereby significantly reducing the formation of soot, particulates and CO. These phenomena take place without adversely affecting the yield for the combustion process.
Several solution have been proposed in attempts to add water to liquid fuel at the time of use, that is to say just before the fuel is injected into the combustion chamber, or directly into the chamber itself. However, these solutions require modifications to be made to the structure of the combustion apparatus and are not capable of achieving optimum dispersion of the water in the fuel, which is an essential requisite for obtaining a significant reduction in pollutants without compromising the calorific yield for the process.
Thus, the most promising and numerous efforts made hitherto were directed towards the formulation of emulsions between liquid hydrocarbons and water in the presence of emulsifiers (surfactants) for the purpose of uniformly dispersing the water in the hydrocarbon phase in the form of micelles of the smallest possible size.                For example, patent EP-A-475 620 describes microemulsions of a diesel fuel with water, which contain a cetane improver and an emulsifying system comprising a hydrophilic surfactant and a lipophilic surfactant. These surfactants are selected from ethoxylated C12C18 alkylammonium salts of a C9—C24 carboxylic or sulphonic acid: the hydrophilic surfactant contains at least six ethylene oxide units, while the lipophilic surfactant contains less than six ethylene oxide units.        
Patent EP-A-630 398 describes a fuel in the form of an emulsion consisting of a hydrocarbon fuel, from 3 to 35% by weight of water and at least 0.1% by weight of an emulsifying system consisting of a sorbitan oleate, a polyalkylene glycol and an ethoxylated alkylphenol.
Patent application WO 97/34969 describes an emulsion between water and a hydrocarbon, for example a diesel fuel. This emulsion is stabilized by adding an emulsifier consisting of a sorbitan sesquioleate, a polyethylene glycol monooleate and an ethoxylated nonylphenol. This emulsifier has an overall HLB (hydrophilic-lipophilic balance) value of between 6 and 8.
A process for producing a stabilized emulsion of a liquid fuel and water is described in patent EP-A-812 615. This process involves preparing a first emulsion by mixing the fuel, the water and a surfactant, and subsequently mixing the emulsion thus obtained with more water to give the final emulsion. The emulsion is stabilized using a hydrophilic surfactant or a lipophilic surfactant, or a mixture thereof. Lipophilic surfactants which can be used are fatty acid esters of sorbitol, for example sorbitan monooleate, while hydrophilic surfactants which are suitable for this purpose are fatty acid esters of sorbitol containing a polyoxyalkylene chain, for example polyoxyethylene sorbitan trioleate. Further stabilization of the emulsion can be obtained by adding ethylene glycol or a polyethylene glycol.
Patent application WO 92/19701 describes a process for reducing the emission of NOx from a gas turbine, in which an emulsion of water with a diesel fuel is used. The emulsion is stabilized by adding an emulsifier selected from: alkanolamides obtained by condensing an alkylamine or hydroxyalkylamine with a fatty acid; and ethoxylated alkylphenols. The emulsifier preferably has an HLB value of less than or equal to 8. Physical stabilizers such as waxes, cellulose derivatives or resins can be added to improve the stability. As described in patent application WO 93/07238, the above emulsion can be further stabilized by adding a difunctional block polymer with a primary hydroxyl end group, in particular a copolymer containing propylene oxide/ethylene oxide blocks.
On the basis of the Applicant's experience, the possibilities of success in the use of fuels in the form of an emulsion between water and a liquid hydrocarbon are mainly associated with the possibility of replacing a conventional liquid fuel with an emulsified fuel without the need for any structural changes to the combustion apparatus and without adversely affecting the correct functioning of this apparatus.
In particular, the fuel in emulsion form requires high stability over time in a broad temperature range (for example for at least three months under normal storage conditions, i.e. between −20° C. and +50° C.), so as to avoid, during residence in tanks, the formation of a water-rich phase which tends to become deposited at the bottom of the tank. Feeding this aqueous phase into the combustion chamber would bring about a considerable impairment in the performance level of the engine, or even permanent damage thereto.
In addition, the Applicant has found that the addition of emulsifiers to improve the stability of the emulsion can lead, during combustion, to the formation of carbonaceous deposits which adhere to the internal surface of the combustion chamber and to the injectors. This phenomenon can adversely affect the running of the engine, as a result of which frequent maintenance is necessary to remove these deposits.